Method for preventing explosions

ABSTRACT

In a process in which a gas comprising oxygen is bubbled through a liquid reaction medium contained in a reaction vessel, with a flammable or potentially flammable vapor being contained in the ullage space of the vessel with a resultant explosion hazard, the explosion hazard is minimized by continuously spraying a liquid comprising predominantly at least one component of the reaction medium into the ullage space so as to maintain a suspension of liquid droplets throughout the ullage space.

United States Patent Gordon [451 Sept. 19, 1972 METHOD FOR PREVENTING EXPLOSIONS [72] Inventor: William E. Gordon, Pittsburgh, Pa.

[73] Assignee: Celanese Corporation of America,

New York, N.Y.

[22] Filed: Feb. 16, 1970 [21] App1.No.: 14,855

Related US. Application Data [62] Division of Ser. No. 514,653, Dec. 17, 1965,

abandoned.

[56] References Cited UNITED STATES PATENTS 3,131,212 4/1964 Biller ..260/497 R X 3,253,020 5/1966 Schaeffer ..260/497 A FOREIGN PATENTS OR APPLICATIONS 860,212 12/1952 Germany ..260/497 A 1,185,604 l/l965 Germany ..260/497 A Primary Examiner-James A. Patten Assistant Examiner-Richard D. Kelly Attorney-Stewart N. Rice, Marvin Turken, Kenneth A. Genoni and Ralph M. Pritchett [57] ABSTRACT In a process in which a gas comprising oxygen is bubbled through a liquid reaction medium contained in a reaction vessel, with a flammable or potentially flammable vapor being contained in the ullage space of the vessel with a resultant explosion hazard, the explosion hazard is minimized by continuously spraying a liquid comprising predominantly at least one component of the reaction medium into the ullage space so as to maintain a suspension of liquid droplets throughout the ullage space.

2 Claims, No Drawings METHOD FOR PREVENTING EXPLOSIONS This is a division of copending application Ser. No. 5 l4,653, filed Dec. 17, 1965, now abandoned.

This invention relates to the handling of potentially explosive gases. More particularly it relates to the suppression of flame propagation in gaseous mixtures containing oxygen and an oxidizable gas. Specifically it relates to the enhancement of operating safety in chemical processes in which potentially explosive gaseous mixtures of oxygen and organic compounds are evolved from liquid reaction media.

In organic chemical technology there are many processes in which molecular oxygen is employed to;

oxidize an organic feedstock. There are two basic techniques, the first being to conduct the oxidation with both oxidant and organic feedstock being in the vapor phase and the second, referred to as liquid-phase oxidation, being to pass molecular oxygen, pure or mixed with a diluent, a coreactant, or both, into a liquid which is a reactant, a reaction solvent, or both, and which may contain an oxidation catalyst.

Vapor-phase oxidation is exemplified by the catalytic oxidation of methanol to produce formaldehyde. Liquid-phase oxidation in a simple form is exemplified by the oxidation of liquid acetaldehyde by a gas containing molecular oxygen to produce acetic acid.

More complicated liquid-phase oxidations, some of which can be described as oxidative couplings, are found in processes in which both oxygen and a coreactant, typically an olefin, are passed into a liquid containing an oxidation catalyst. The liquid may be simply a reaction solvent (e.g., water) or it may be itself a reactant as well as a solvent, reacting with the oxygen and the other reactant (e.g., an alcohol or a carboxylic acid).

Liquid-phase oxidations in particular are subject to a safety hazard in that there is danger of the accumulation of potentially explosive vapors in the ullage space of the reaction vessel or in other spaces containing vapors evolved from the reaction vessel. This is due to the fact that in practically all liquid-phase oxidations a portion of the oxygen passes through the reactor unconverted even when operation is normal. Under normal operating conditions the formation of explosive gas mixtures is avoided by so controlling the reaction system that the evolved vapors remain safely on the socalled rich side of the explosive range; that is, the concentration of oxygen is maintained below the maximum safe oxygen concentration," defined as the oxygen concentration, in a fuel-rich gas, below which flame propagation will not occur and above which flame propagation can occur if the gas is ignited. Several conditions, however, can result in the formation of an explosive mixture. For example, the poisoning of an oxidation catalyst can result in a diminished oxygen uptake, with a resulting increase in oxygen content in the vapors evolved from the reaction. A similar result can obtain if the reaction temperature falls below a critical point, resulting in a diminished reaction rate, or if a failure of oxygen control devices allows too high a rate of oxygen input, or if a failure of gas-dispersion devices allows oxygen to pass through the reaction liquid in bubbles too large to allow efficient absorption and consequent consumption.

The result of any of the above-described malfunctions can be the formation of an explosive mixture of vapors, either in the oxidation reactor or in downstream equipment containing vapors evolved from it. Particularly since commercial-scale liquidphase oxidation equipment generally contains large process inventories and is frequently under substantial pressure, the consequences of an explosion can be disastrous.

In the existing art such explosions are usually prevented by utilizing a high'degree of automatic control to (a) maintain safe reactor conditions of temperature, pressure, and reactant flow rates, and (b) monitor the oxygen content of the vapors continuously with instruments which will automatically cut off the oxygen flow and replace the oxygen with large quantities of an inert gas if an explosive mixture begins to form. Such an automatic shutdown system for a reactor in which ethylene and oxygen are reacted in an aqueous medium to produce acetaldehyde is mentioned in Chemical Engineering, Dec. 9, 1963, pages -152. These methods are all effective so long as the control instruments involved are in good working order, but the art continues to seek additional safeguards to supplement the action of the control instruments and provide protection in event of their malfunction.

It is an object of this invention to provide a method for suppressing explosions in enclosed spaces containing potentially explosive mixtures of oxygen and flammable gases.

It is a further object of this invention to provide a method for increasing the concentration of oxygen allowable in a given gas mixture before danger of a serious explosion exists.

It is an additional object of the invention to provide means for improving the operating safety of chemical processes in which oxygen is reacted with combustible organic materials and in which potentially explosive vapors containing oxygen and combustible gases are evolved in an enclosed space.

Other objects of this invention will be apparent from the following detailed description and claims.

One approach to preventing explosions in process vessels, fuel tanks, etc. which contain potentially explosive mixtures has been the invention of ingenious systems whereby the beginning of an explosion, as indicated by, for example, a slight rise in pressure, is sensed by delicate instruments which immediately activate mechanisms which flood the threatened space with a fire-extinguishing fluid, e.g., a non-combustible gas or liquid having fire extinguishing properties, so rapidly that the incipient explosion is stopped before attaining dangerous strength. Such systems, although extremely effective over a wide range of conditions, are essentially for use on an emergency and intermittent basis, rather than routinely and continuously, and require recharging with the extinguishing fluid after each use. Burning liquids, as distinguished from exploding gases, can also be quenched by pumping cool liquid from the bottom of the containing vessel to the burning surface so rapidly as to stop the fire. Another method, described in US. Pat. No. 2,757,744, entails spraying a volatile fuel into a fire zone under conditions such that it evaporates and generates enough vapor that the oxygen concentration is lowered below the maximum safe concentration.

The present invention differs from the above in that (a) it operates continuously with materials which are inherently present and which need not even be nonflammable, and (b) it does not entail altering the composition of the vapors. its purpose is not to extinguish explosions under all conditions, but rather to minimize explosion hazard by raising the effective maximum safe oxygen concentration.

In accordance with the present invention, process equipment containing a mixture of oxygen and combustible gases, in which the concentration of oxygen is at or near the lowest level at which flame propagation is possible, is protected from explosion by spraying a liquid, which can be a material already present in the system and which can be combustible, into that portion of the equipment which contains a vapor phase, and continuously maintaining droplets of the liquid suspended in the vapor phase. It has been discovered that the use of this technique makes possible the safe handling of mixtures of oxygen and flammable gases having an oxygen concentration several percentage points in excess of the normal maximum safe oxygen concentration characteristic of the mixture in question, the normal maximum safe oxygen concentration being defined as that characteristic of the mixture when it is not protected by the presence of the liquid droplets.

When gas mixtures having an oxygen content near, but definitely above, the maximum safe oxygen concentration as defined herein are filled with a suspension of liquid droplets and then exposed to an ignition source such as an electric spark, it has been found that, although a small temperature rise in the immediate vicinity of the spark may occur, indicating that localized ignition has taken place, the flame does not propagate through the gas as a whole and there is no explosion. in the absence of the spray the gas ignites, flame propagates throughout the entirety of the gas volume, and an explosion occurs. The effect is observed whether the droplets are of flammable or nonflammable liquids and is of substantial magnitude, expressed quantitatively as percentage points of oxygen concentration which can be tolerated, without explosion, above the normal maximum safe oxygen concentration.

The effect is limited, however, within limits which for a given system can be determined by test procedures which will be described. Above a certain oxygen concentration, characteristic of each gas mixture, the liquid spray may not prevent flame propagation and explosion if the gas is ignited, so the reliable application of this invention presupposes a system in which the composition of the gas mixture is being controlled within a range not greatly exceeding normal maximum safe oxygen concentration. The invention is primarily a protective supplement to such control rather than a complete replacement for it.

The industrial application of the invention may be described broadly as follows:

Substantially all processes in which a gas containing molecular oxygen is brought into contact with a liquid phase for the purpose of carrying out a reaction between oxygen and an organic material present in the liquid phase employ one of two types of reaction system:

a. a vessel containing a liquid into which the oxygencontaining gas is bubbled, a portion of the oxygen becoming absorbed into the liquid and the remainder passing through the liquid and becoming disengaged therefrom into a vapor-filled ullage space in the upper portion of the vessel, or

b. a gas-liquid contacting device which operates substantially full of an intimate mixture of gas and liquid (i.e., with little or no ullage space), from which the gasliquid mixture is discharged into a separate gas-liquid separator. This type of system is exemplified by socalled pipe reactors in which gas and liquid are caused to flow together through elongated vessels, commonly placed vertically and often containing internal orifices, baffles, or other means of promoting turbulence.

All such systems, regardless of variations in details of arrangement and operation, are characterized by the presence, in one location or another, of a plenum which is filled with a gas containing oxygen and varying quantities of organic vapors which have either been swept out of the liquid phase by the oxygen or which comprise a gaseous organic reactant, such as ethylene, which has itself been bubbled through the liquid phase along with the oxygen. The plenum may constitute the ullage space of the reactor, bounded by the reactor walls and the surface of the liquid contained in the lower portion of the vessel, it may consist of the gascontaining portion of a vapor-liquid separator following the reactor, or it may consist of several such spaces interconnected and having in common only the fact of being filled with a mixture of oxygen and combustible gases evolved from the oxidation reactor.

However this plenum is situated, it is subject to the danger of explosion if the composition of the gas contained within it is within the explosive range. The likelihood of this occurrence depends somewhat upon the nature of the individual reaction system, but those systems in which oxygen and a volatile olefin, such as ethylene or propylene, are caused to react in the presence of a redox catalyst system containing, typically, palladium and copper compounds, are especially delicate in this respect because a substantial excess of oxygen is maintained in the gases bubbled through the reaction zone, resulting in the passage of substantial quantities of oxygen into the aforementioned plenum along with large quantities of combustible vapors. Such systems are typically protected in the existing art by (a) extensive instrumentation to control the composition of the gases and keep it out of the explosive range (i.e., as a practical matter, on the fuel-rich side of the explosive range), (b) back-up instrumentation to provide rapid automatic shut-down and inert gas purging in the event the composition-control devices fail, and (c) design of the equipment to contain a high over-pressure if both (a) and (b) fail and ignition should occur. In the previously referred-to article in Chemical Engineering of Dec. 9, 1963, for example, all three expedients are employed in an acetaldehyde plant, including the design of the reaction system to withstand eight times the normal operating pressure.

The present invention can be applied to all such systems, because they are all characterized by the presence of the gas-filled plenum and the availability of liquids (i.e. the liquid reaction media or major components thereof) which can be employed as protective sprays, the introduction of which into the plenum cannot contaminate the gases since the gases have already been contacted with the same liquids in the ordinary course of operation. A straightforward method can be employed to determine the degree of protection afforded by application of the invention to a particular gas mixture. it comprises:

a. equipping a suitable vessel with a rapid venting device such as a bursting disc, means for measuring and controlling the temperature and pressure of the contents of the vessel, means for filling the vessel with the gas mixture being tested, means for changing the composition of the gas mixture in the vessel by adding measured increments of any of the component gases but especially oxygen, means for mixing the gases contained in the vessel, means for measuring the oxygen content of the gases within the vessel, means for injecting a liquid spray into the upper portion of the vessel from spray heads of known characteristics spaced in a regular and known pattern, means for controlling the composition, temperature, and flow rate of the liquid injected into the vessel through the spray heads, means for draining accumulated liquid from the bottom of the vessel, and a means for ignition, such as a sparking device, located preferably near the center of the vessel and shielded from contact with gross quantities of the liquid spray;

b. filling the vessel with a gas of a composition equivalent to that expected to obtain in the plenum which is to be protected by application of the invention; I

c. adjusting the pressure and temperature of the gas contained in the vessel to the pressure and temperature expected to obtain in the plenum to be protected, maintaining this pressure and temperature throughout the remaining steps of the test procedure;

d. injecting into the vessel through the spray heads a liquid the efficacy of which in applying the invention to the system in question is being tested, the rate of injection and pressure drop through the spray heads being controlled at a set value such as to provide a known degree of dispersion in the resulting spray in accordance with known operating characteristics of the spray heads being employed;

e. continuing the injection of the spray until a steadystate concentration of spray droplets is attained in the vessel, typically for a matter ofa few seconds;

f. while the spray is still being maintained in the vessel, activating the ignition source while observing the pressure in the vessel for evidence of flame propagation as indicated by a pressure rise;

g. injecting a known amount of oxygen into the vessel, while mixing it with the gases already present therein, until the oxygen content of the gas mixture has been increased by a measured finite increment, advantageously about one-half to two volume percentage points;

repeating and above;

i. repeating the cycle of steps (c) through (h) until an oxygen concentration has been attained at which, upon activation of the ignition source, a sharp rise in vessel pressure indicates that the contents have ignited and that flame propagation has occurred. The oxygen concentration in the gases at this point is that at and above which the invention does not prevent flame propagation and consequent danger of explosions at the spray rate being employed in the test.

The rate of spraying, i.e., the concentration of liquid droplets maintained in the gas, has some effect on the degree of protection afforded by the application of the invention, but above a certain minimum, which can be determined by repeating the above-described test procedure at various spray rates, its effect is not criti cal. A practical and effective spray concentration at which explosions may be suppressed by the application of this invention is about 0.2-0.5 part of suspended liquid droplets per part of gas, by weight, so a spraying rate calculated to provide this concentration in the test chamber is a practical and realistic rate for use in testing applications of the invention. The spray heads should be selected to deliver droplets which are largely in the 50 to 500 micron diameter range. Fine droplets are more effective than large, but 50 to 500 micron droplets have been found effective in practicing the invention and can be produced easily with commercially available spray heads. 7

Equipment which is generally applicable in testing the application of this invention and which was actually employed in the case of the examples described herein is as described below:

First the test vessel itself should be sufficiently large to minimize wall effects and to accommodate the output of at least one spray head of a commercially available type. Accordingly, the vessel employed in testing this invention consisted of a steel sphere having a volume of approximately 6.7 cubic feet and a cross sectional area, measured at a great circle, of approximately 5 square feet. The vessel had a working pressure of 500 lbs. per square inch at 650F. Electric strip heaters were affixed to the outer wall in order to maintain the vessel at elevated temperatures, and glass fiber insulation was used to minimize heat losses. Temperature measurement and control were achieved by a thermocouple located in a thermowell extending into the vessel and attached to a conventional temperature recorder-controller. The vessel was equipped with an 8-inch diameter bursting disc of stainless steel 0.024 inch thick. A spark gap, connected through a switch to means for energizing it to produce an electric spark having an energy content of approximately joules, was mounted near the geometric center of the vessel. Short-circuiting of the spark gap by liquid spray droplets was prevented by mounting over the spark gap a shroud consisting of a one pint cylindrical plastic container, disposed horizontally, with its lower portions cut out to allow insertion of the spark gap and free circulation of the gaseous contents of the vessel around the spark gap. The thermocouple well previously referred to could be positioned either within the shrouded enclosure around the spark gap or outside it. When positioned inside the shrouded enclosure, the thermocouple was able to sense localized temperature rises indicating localized ignition, even at times when because of the action of the spray no flame was propagated through the remainder of the vessel.

Means for sensing explosion pressures within the vessel consisted of a strain gauge transducer mounted on a vessel port cover and recording on a oscillograph.

Gas mixtures were made up in the sphere by injecting each gas in turn from a high-pressure cylinder through a tube which penetrated to near the center of the sphere and which had a small orifice nozzle on the end. The resulting high-velocity jet stirred the contents of the vessel and aided mixing. Gas formulations in the vessel were prepared on the basis of manometry employing incremental partial pressures of the various gases as read from a precision Bourdon gauge. Once the initial formulation had been prepared for a given test, subsequent additions of small increments of oxygen were made by using injection from a small pressure bottle having a known volume about 2 percent that of the sphere rather than by manometry.

Liquids to be injected into the vessel through the spray heads were contained in pressure vessels equipped with strip heaters, insulated, and connected to cylinders of nitrogen in order to provide the requisite injection pressure.

Piping connections for supplying liquid from the above described pressure vessels were led through the walls of the spherical vessel at four points spaced equidistantly around the sides of the vessel approximately 3 inches below its equator. Each such connection was provided with a solenoid valve outside the vessel. Each of these connections, after passing through the wall of the vessel, terminated approximately three inches inside the wall with a spray head, so mounted as to discharge toward the center of the vessel in a direction approximately 45 above the horizontal. The spray heads were interchangeable, and any one or all four could be actuated as desired.

The following examples are given to illustrate the invention further.

EXAMPLE 1 The vessel described above was filled with a gas mixture comprising 70 mole percent ethylene, mole percent nitrogen, and 20 mole percent carbon dioxide. The pressure and temperature were adjusted to 450 lbs. per square inch absolute and 230F, respectively. The sparking device was not shrouded. Oxygen was then blended with the gases until the oxygen content of the resulting mixture was 5.5 mole percent. The sparking device was actuated and there was no explosion. The oxygen concentration was then increased to 5.886 mole percent, and again a spark resulted in no explosion. Similarly, no explosion occurred at 6.285 mole percent oxygen, but after the oxygen content had been increased to 6.645 percent, a spark resulted in an explosion which broke the bursting disc and vented the vessel.

EXAMPLE 11 Example I was repeated but with the difference that the shroud was installed over and partially enclosing the sparking device. In addition a thermocouple was positioned under the shroud with the sparking device. At 7 mole percent oxygen a spark produced no explosion, but a temperature rise of 2F. was observed in the space enclosed by the shroud. At 7.45 mole percent oxygen a spark caused an explosion, which broke the bursting disc and vented the vessel.

EXAMPLEllI The test vessel was equipped with two Spraco Type 1 158M spray nozzles, manufactured by Spray Engineering Co., Burlington, Massachusetts. At 200 lbs. per square inch pressure drop, each of these nozzles delivers through an orifice 0.28 inch in diameter, about 10 gallons per minute in a open cone. The vessel was filled with the same gas mixture employed in the preceding example and at the same temperature and pressure. Oxygen was admitted until the gas mixture contained 10 mole percent oxygen. The two sprays were then actuated for 3 seconds feeding liquid acetic acid at 210F. at 200 lbs. per square inch pressure drop through the nozzled. Immediately before cessation of the spraying the sparking device was actuated. N0 explosion occurred, but a small temperature rise in the shrouded enclosure indicated that localized ignition had taken place, flame propagation into the rest of the vessel having been prevented by the spray. The oxygen content was then increased to 13.32 mole percent and the spray actuated as before. A spark produced a small .temperature rise within the shrouded enclosure, but no explosion. Upon increasing the oxygen concentration once again, to 16.42 mole percent, followed by actuation of the sprays and sparking device as before, an explosion occurred and broke the bursting disc.

EXAMPLE 1V Example 111 was repeated except that the temperature was increased to 248F. At 13 mole percent oxygen the mixture did not explode when sparked but a slight temperature rise within the shrouded enclosure indicated that localized ignition had occurred. At 14.86 percent oxygen, the mixture ignited when sparked and the bursting disc was broken, indicating propagation of the flame throughout the vessel.

EXAMPLE V The same equipment and operating procedures were employed as in Example 1V, except that the sprays were operated at lbs. per square inch rather than 200 lbs. per square inch pressure drop. At 1 1.0 percent oxygen there was a slight temperature rise within the shrouded ignition zone indicating localized ignition, but no explosion to indicate flame propagation through the vessel. The same occurrence was observed at oxygen concentrations of 12.4; 13.7; 15.0; 16.5; and 17.1 mole percent. At 19.1 mole percent oxygen there was an explosion and the bursting disc was broken.

EXAMPLE V1 The conditions and procedures of Example V were repeated, except that only one of the spray nozzles was used. The initial mixture contained 1 1 percent oxygen, and there was an explosion when it was sparked, breaking the bursting disc.

EXAMPLE VII The vessel was filled with the same gas mixture as before, and oxygen was added to make a mixture containing 7 mole percent oxygen. Acetic acid was sprayed into the vessel for 13.2 seconds through two Spraco Type 1158M nozzles at 200 lbs. per square inch pressure drop. After a 17-minute stabilization period at 248F., the mixture was sparked and no ignition occurred. The oxygen content was increased to 8.01 mole percent, and the mixture was sparked again. The mixture ignited and the bursting disc was broken. The purpose of this experiment was to determine whether the presence of acetic acid vapor had an appreciable effect on the maximum safe oxygen concentration. It was concluded that any effect was small.

EXAMPLE VIII The vessel was equipped with three Spraco heads each consisting of a cluster of seven Spraco Type 1529 Pin-jets positioned in a stainless steel housing. These jets employ orifices 0.064 inch in diameter and produce a very fine spray. The nozzles were positioned at the equator of the vessel as in previous examples, directed upward at an angle of 45 from the horizontal. They were positioned 90 apart, thus leaving a gap at one position on the equator where a fourth nozzle was located but not operating during this test. The vessel was filled with the same gas mixture employed in previous examples, at 230F. and 435 lbs. per square inch absolute. Oxygen was then introduced to make a gas mixture containing 8 mole percent oxygen. Injection of acetic acid at a pressure drop of 200 lbs. per square inch was then begun through the three spray nozzles. Two seconds later, with the spray still operating, the mixture was sparked, and one second after sparking the spray was discontinued. The mixture did not ignite. The oxygen content of the gases was then increased to l 1.34 mole percent and the spray and ignition attempt were repeated as before. No explosion (flame propagation) occurred, but a 4F. temperature rise was noted in the shrouded enclosure at the time of sparking indicating localized ignition. The same procedure was repeated with an oxygen content of 13.98 mole percent. Once again no explosion occurred upon sparking, but an 8F. temperature rise was noted in the shrouded enclosure, indicating localized ignition.

The foregoing examples all deal with systems comprising a plenum filled with a stagnant gas. In most industrial applications, the gas will not be stagnant but will be flowing through the plenum and out through an opening commonly located at or near the top. For such situations, the test methods employed in the preceding examples yield a valid indication of explosion protection to be expected. That is, explosion suppression indicated in tests made with a stagnant system will also obtain in a flowing system. This is due to the fact that in a flowing system there will be some holdup of the finer droplets the settling velocity of which is near or less than the average velocity at which gases are rising through the plenum in question. The net result is that in a flowing system the concentration of droplets is increased somewhat over that obtaining in a non-flow system, saving only the fact that, below the level of the spray injection points, the mixture of droplets will have been denuded of those droplets so small as to have a settling rate lower than the upward velocity of the gas stream. Any increase in concentration of droplets increases the degree of explosion protection to be expected.

in commercial application of the invention to a typical situation in which a plenum containing a flowing gas (typically an upwardly flowing gas) is to be protected with a liquid spray, the primary considerations in the design of the requisite equipment are as discussed below.

First, an array of spray nozzles is to be installed, more or less in a horizontal plane which is a cross section of the plenum at or near its upper extremity. if the plenum has a constriction at its upper extremity, such as the inwardly tapering head of a pressure vessel, the nozzles would be placed below the constricted zone or else a non-horizontal array would be adopted, some of the nozzles being in the constricted zone and others, at a lower level, spaced over the peripheral portions of the plenum which are not more or less directly below those nozzles which are located at higher levels and in the constricted zone. Those portions of the plenum situated below the nozzles will contain a dispersion of liquid droplets in a concentration which can be determined and controlled by methods which will be discussed below. Above the nozzles, that is between the nozzles and a vapor outlet presumed to be located at the top of the plenum, the plenum will be filled with a dispersion of those droplets which are too fine to settle downward through the upwardly flowing gas. The nozzles should be spaced uniformly and on centers close enough that in the plenum below them there will be no spaces of any size which are void of spray. As an example, an arrangement which has been found satisfactory comprises, in a vertical cylindrical vessel having a cross sectional area of 104 square ft., 50 spray nozzles, each having a liquid delivery capacity of about 10 gallons per minute at 200 lbs. per square inch pressure drop, arrayed on 1 foot centers in the head of the vessel. A gas having a composition similar to that described in the Examples given herein and at a temperature and pressure similar to that described in the examples moves upwardly in this vessel at a rate of approximately 10 centimeters per second. Other satisfactory arrangements exist, this being described merely as one practical illustration.

The concentration of liquid droplets which will be attained in a given plenum depends upon the following:

1. the number of nozzles employed,

2. the liquid discharge rate of each nozzle at the nozzle pressure drop chosen for the installation,

3. the droplet size distribution characteristic of the nozzle at the pressure drop being employed and with the liquid being sprayed therethrough,

4. the velocity of the gas through the plenum, and whether this velocity is upward or downward,

5. whether the distribution is being determined at a point above or below the nozzle.

Of the above listed factors, the minimum droplet concentration to be maintained can be determined by the experimental procedure described herein. A practical and reasonably attainable magnitude for this factor, under conditions described in the examples herein, is about 0.2 to 0.5 pound of liquid droplets per pound of vapor.

The first step in sizing the spray equipment to provide the desired droplet concentration is to determine the droplet size distribution which will be characteristic of the nozzle to be employed, with the liquid which is to be sprayed, at the nozzle pressure drop which is to be employed. This subject is treated in pages 66-68 of Perrys Chemical Engineers Handbook, Fourth Edition, Section 18. Next, settling velocities in stagnant gas are computed for liquid droplets in the range of droplet sizes produced by the nozzle in question, employing, for example, methods described on pages 61 and 62, Section 5, in Perry: Handbook. It is to be noted that the droplets are not all of the same size, so that it is desirable to repeat the calculation for each of several fractions in the droplet size range produced by the nozzle. Those droplets the stagnant gas settling velocity of which is less than the linear gas velocity through the vessel will rise with the gas rather than fall downward through the vessel. Droplets larger than this size will fall downward at a net settling velocity equal to their stagnant gas settling velocity less the gas velocity. The steady state condition with respect to droplets of a given size in the zone beneath the nozzles is expressed in the relation: concentration of droplets times net settling velocity=rate at which droplets are discharged from the nozzles per unit of cross sectional area. Above the nozzles, only those droplets whose stagnant gas settling velocity is less than the linear gas velocity will be present, theoretically, so that the liquid droplet concentration in this part of the plenum is simply computed from the gas flow rate and the weight of droplets discharged from the nozzles per unit time having diameters such that their net settling velocities are less than zero.

In vessels, such as certain gas-liquid separators, in which vapor flow is downward, design procedures are similar to the above except that the net settling velocity of the liquid droplets is the sum of, rather than the difference between, their stagnant gas settling velocity and the linear gas velocity.

The invention is broadly applicable to all processes in which a gas containing molecular oxygen and flammable gases, having a composition near the explosive limit at the fuel-rich end of the explosive range, is evolved from a liquid reaction medium. Specific industrial applications include the following:

a. The production of carbonyl compounds by the reaction of olefinic compounds and oxygen in aqueous solution containing a redox catalyst system. This is illustrated in the Chemical Engineering article previously referred to herein and in a paper by Smidt et al. published in Angewandte Chemie 71, 176-182(1959). These processes are considered to proceed by way of complexes of the olefinic raw materials with precious metals, particularly palladium. Practically any olefin having at least one hydrogen atom on each carbon atom of the double bond can be converted to a carbonyl compound by these methods, provided the configuration of the molecule is such that there is no steric hindrance towards complexation. The carbonyl group will appear on that carbon atom to which the anion would add in ionic addition of acids in accordance with Markovnikovs rule. Both mono-olefins and polyolefins can be converted to monoand polysubstituted carbonyl compounds, and both simple and substituted hydrocarbons can be employed. Simple olefinic hydrocarbons generally form the corresponding ketones, or, as in the case of ethylene, the aldehyde, while certain substituted olefins undergo decomposition reactions such as dehydrohalogenation or decarboxylation while forming the carbonyl derivatives of the residual moiety.

as follows:

Olefinic feedstock Carbonyl product Ethylene Propylene l-Butene l-Pentene l-Hexcne l-Heptene l-Octane l-Nonene l-Decenc 1,3 Butadiene 1,4 Pentadiene Cyclopentene Cyclohexene lndene Styrene Alylbenzcnc Acrylic acid Crotonic acid a,fi-Pcntenoic acid afi-Hexenoic acid a, B-Heptenoic acid a,fi-Octenoic acid Methacrylic acid Tiglic acid Bfi-Dimethylacrylic acid Cinnamic acid Sorbic acid Maleic acid ltaconic acid a-Methoxycrotonic acid Allyl alcohol Crotonaldehyde p-Methoxy styrene Anethole lso-safrole Vinyl acetate lospropenylacetate Vinyl chloride Vinyl bromide l-Bromol-propenc LBromo-l-butene l-Bromo-l-pentenc l-BromoJ-hexene l-Bromo-l-heptene B-Bromostyrene l-Chloro-Z-methyl, l -propenc I -chloro-Z-methyLZ-propene 2-Chloro-l-propene a-Chlorostyrene a-Chloro-B-mcthylstyrene l-Ch|oro-3-phcnyl l-propcne 2-Chl0ro-3-phcnyl-l-propene Allyl chloride Allyl bromide l-Bromo-4-pentene p-Chlorostyrene 2,3-Dibromo-l-propene l,3 Dichloro-l-propene a-Bromoacrylic acid a Chlorocrotonic acid a-Bromo-a,B-pentenoic acid a-Bromo-a,fi-hexenoic acid aBromo-a,B-heptenoic acid a-Bromo-a,B-octenoic acid a-Bromo cinnamic acid p'Chloro cinnamic acid Acetaldehydc Acetone Methyl ethyl ketone n-Propyl methyl ketone n-Butyl methyl ketone n-Amyl methyl ketone n-Hexyl methyl kctone n-Heptyl methyl ketone n-Octyl methyl ketone Crotonaldehyde Z-Pentenc, l-al Cylcopentanonc Cyclohexanone Beta-indanonc Acetophenonc Benzyl methyl ketone Acetaldehydc Acetone Ethyl methyl ketone n-Butyl methyl ketone n-Butyl methyl ketone n-Amyl methyl ketone Propionaldehyde Methyl ethyl ketone Acetone Acetophenone Ethylideneacetone Pyroracemic acid Succinic acid Acetone Acrolein Triacetylbenzene p-Methoxyacephenone p-Methoxyphenylacetonc 3,4-Methylenedi0xybenzyl methyl ketone Acctaldehyde Acetone Acetaldehyde Acetaldehydc Acetone Ethyl methyl ketone n-Propylmethylketone n-Butylmethylketone n-Amylmethylketone Acetophenone Acetaldehyde Alphamethylacrolein lsobutyraldehyde Alphamethylacrolein Acetone Acetophenone Propiophenone Benzyl methyl ketone Benzyl methyl ketone Methylglyoxal Mcthylglyoxal -y-Bromopropyl methyl ketone p-Chloroacetophenone Methylglyoxal Methylglyoxal Acetaldehyde Acetone Ethyl methyl ketone n-Propyl methyl ketone n-Butyl methyl ketone n-Amyl methyl ketone Acetopheonone p-Chloro acetophenone The application of the invention to the reaction systems described above comprises spraying either water or the aqueous reaction medium, which is an aqueous solution of the carbonyl compounds being produced and the redox catalyst, commonly a mixture in which are dissolved a source of acyloxy groups and a redox catalyst system similar to that employed in (a) above comprising a noble metal of Group VIII of the periodic table and a salt of a varivalent metal, such as cupric chloride. The liquid reaction medium is maintained in a-substantially anhydrous condition, although .some water, i.e., up to about percent, can be tolerated. In these reaction systems the oxygen is bubbled through the liquid phase, while the olefin, depending upon its volatility characteristics and the concentration at which it is fed into the reaction system, may be either bubbled through the reaction medium as a gas or mixed thereinto as a liquid. In either event, substantial quantities of the olefin in the vapor phase will be in admixture with that portion of the oxygen which has bubbled through the liquid phase unabsorbed. The product of these reactions consists of a carbonyl derivative of the olefinic compound, formed in accordance with the principles described in (a) above, and ester derivatives of the carboxylic acid, the alkoxy portion of the esters being derived from the olefinic compound employed in the reaction. For example, vinyl acetate is formed from ethylene and acetic acid while isopropenyl acetate is formed from propylene and acetic acid. The unsaturated esters formed in these reactions are in part vinyl type esters and in part allyl type esters. Also formed as co-products are glycol esters and gem-diol esters, to an extent which can be modified somewhat by control of reaction conditions. As in the case of (a) above, and subject only to the limitations mentioned in (a) above, that is that there be at least one hydrogen on each carbon atom attached to the double bond, a very wide range of olefinic compounds can be employed as feedstock in these processes. Likewise, a very wide range of carboxylic acids can be employed. Both aliphatic and aryl aliphatic acids are suitable, and mono-, di-, and tricarboxylic acids can be employed. In a simple example, for instance, propylene can be reacted with acetic acid to form a mixture of products containing n-propenyl acetate, isopropenyl acetate, propylidene acetate, isopropylidene acetate, 1,2-propylene glycol diacetate, and acetone. French Patent No. 1.324.029 describes the production of acetaldehyde together with vinyl esters of carboxylic acids by these methods, while British Patent Nos. 975,683 and 975,709 describe similar operations with a broader range of olefinic raw materials. French Patent No. 1.308.724 describes the use of similar methods in reacting olefins with a broad range of carboxylic and dicarboxylic acids including aromatic-substituted acids.

As has been noted, substantially any olefin is operable in this invention provided that it has a structure such that no steric hindrance is offered to the reaction. Olefins having substitutents near the point of unsaturation will react, although in certain instances there may be a degradation side reaction. For example, a-B unsaturated carboxylic acids will tend to decarboxylate.

Suitable olefins are exemplified by, but not restricted to, simple hydrocarbons such as ethylene, propylene, butenes, octenes, dienes such as butadiene and cyclopentadiene; unsaturated acids such as oleic, linoleic, and linolenic; ethers such as allyl methyl ether; and aromatic substituted olefins such as styrene. Halogen and nitrogen substituted olefins will also react, although the former will normally dehydrohalogenate if the halogen substituent is near the double bond.

Substantially any carboxylic acid can be employedin these processes, although simple monocarboxylic aliphatic acids are currently most important commercially. Vinyl, ethylidene, and ethylene glycol esters of acetic and propionic acids, with acetaldehyde as a co-, product, can be produced by reactions described in French Patent No. 1.308.723. The use of higher molecular weight acids, including dicarboxylic and tricarboxylic acids, can be practiced as described in French Patent No. 1.308.724. Olefins having from three to 20 carbon atoms are reacted with a number of carboxylic acids including acetic, propionic, phthalic, n-hexanoic, adipic, and benzoic. Also usable are acid salts of the dicarboxylic and tricarboxylic acids as well as the same acids in partially esterified form, for example, monoethyl phthalate or diethyl trimesate. High molecular weight acids such as stearic and oleic can be employed. a

As indicated above, a very large number of olefinic compounds and carboxylic acids can be employed as feedstocks in this family of reactions. In addition, substantial variations are possible in the catalyst systems employed, as described in the patents cited above. Additional variations are also introduced by the fact that the liquid reaction media may contain, in addition to the carboxylic acid and catalyst, any of a large number of inert solvents. Regardless of the extremely large number of potential reaction systems described here, however, the invention is universally applicable in enhancing their operating safety by spraying the liquid reaction medium or a component thereof into any plenum which, in the case in question, is filled with a mixture of oxygen and flammable gases evolved from the reaction system.

c. Processes for the reaction'of olefinic compounds with alcohols to produce carbonyl derivatives of the olefinic compound together with acetals, unsaturated ethers, or both. In these processes molecular oxygen and an olefinic compound which may be either gaseous or liquid are reacted with a liquid medium comprising an alcohol, in which is dissolved a redox catalyst system as described in (b) above. The liquid may contain an inert diluent liquid if desired. The carbonyl product is formed from an olefin in accordance with the principles set forth in (a) and (b) above. The other products, the proportions of which are controllable somewhat by various methods not pertinent to a description of the present invention, consist in part of acetals derived from two moles of the alcohol and one mole of the olefin and in part of unsaturated ethers of which the unsaturated portion is derived from the olefin while the remaining portion is derived from the alcohol. These reactions are discussed in Doklady Akad. Nauk. SSSR 133, No. 2, 377-380 (1960). A typical reaction is that of ethylene, oxygen, and an alcohol to form vinyl ethers and acetals. Higher olefins, e.g., up to about 20 carbon atoms, including substituted olefins and polyolefins, can be employed. The alcohols employed can be simple alkanols or polyols, but with simple alkanols the reaction is more straightforward and the product less complex. The application of the invention to these processes comprises spraying into the ullage spaces to be protected either the alcohol or the alcoholic reaction medium.

d. Processes for the liquid phase oxidation of hydrocarbons and other organic liquids. Many processes of current commercial importance are based upon the liquid phase oxidation of hydrocarbons, generally in the presence of catalysts such as chromium, cobalt, bromides, and borates. In many cases a reaction solvent, such as acetic acid, is employed, while in other cases the reaction medium comprises only the hydrocarbon being oxidized together with the oxidation products. Liquid phase oxidations of nonhydrocarbon raw materials, such as acetaldehyde, fall into this same category insofar as the nature of the oxidation reaction system and the applicability of the present invention to them are concerned. These reactions differ from those described above in that the oxidizable gas mixed with oxygen in the plenum normally comprises vapors of the liquid being oxidized rather than a separately added oxidizable compound such as, for example, a volatile olefin. In the normal operation of such reaction systems, explosions are avoided by so controlling the, reaction that substantially all of the oxygen is consumed as it is bubbled through the liquid reaction medium. However, certain conditions can result in a sharply reduced rate of oxygen uptake or a sharply increased rate of oxygen input to the reactor, resulting in the formation of an explosive mixture in the reactor ullage space and associated equipment. Examples of processes in this category include the following: (a) liquid phase oxidation of aliphatic hydrocarbons of about three to twenty carbon atoms to produce a wide range of products including carboxylic acids, ketones, etc.; (b) liquid phase oxidation of aldehydes of about two to eight carbon atoms to produce carboxylic acids; (c) liquid phase oxidations of xylenes in acetic acid reaction solvent to produce phthalic acids, especially of p-xylene to produce terephthalic acid; (d) liquid phase oxidation of cyclohexane to produce one or more of the group consisting of cyclohexanol, cyclohexanone, adipic acid, and hydroxycaproic acid. In each of these processes the explosion hazard in the critical plenum can be minimized by spraying thereinto, in accordance with the practice of this invention, a liquid comprising either the liquid reaction medium or one of the components of the liquid reaction medium.

In all the above it is to be understood that the degree of protection afforded by the application of the invention, expressed in percentage points oxygen concentration above that existing at the normal maximum safe oxygen concentration, will vary from one system to another. The degree of protection obtainable in a given system can be determined by utilization of the test procedures described herein.

It is further to be understood that the foregoingdetailed description is merely given by way of illustration and that many variations may be made therein without departing from the spirit of my invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for conducting a chemical reaction between oxygen, ethylene and acetic acid to form vinyl acetate, which process comprises bringing ethylene and a gas comprising molecular oxygen into contact with a liquid reaction medium comprising predominantly acetic acid containing a source of acyloxy groups and a redox catalyst system comprising a noble metal of Group VIII of the periodic table and a salt of varivalent metal, a portion of said gas becoming absorbed into said liquid reaction medium and the remainder passing through said reaction medium unabsorbed and becoming disengaged therefrom into a plenum filled with a potentially explosive gas comprising oxygen and flammable vapors at a pressure of about 450 pounds per square inch and having an oxygen content near the maximum safe oxygen concentration and below about 18 percent by volume:

the improvement which comprises minimizing the explosion hazard in said plenum by spraying liquid acetic acid thereinto and continuously maintaining droplets of acetic acid distributed in said potentially explosive gas throughout the entirety of said plenum.

2. The improvement of claim 1, wherein the liquid droplets are maintained in the potentially explosive gas in a concentration of at least 0.2 part of liquid per part of gas by weight.

\OIDSL 1007 

2. The improvement of claim 1, wherein the liquid droplets are maintained in the potentially explosive gas in a concentration of at least 0.2 part of liquid per part of gas by weight. 